Mode discrimination apparatus

ABSTRACT

In an optical system such as a ring laser gyroscope cavity, oscillations occur in many transverse modes. The desired modes are allowed to exist by suppressing undesired modes utilizing a light stop apparatus. One light stop is made by treating a dielectric mirror with an electron beam causing a phase change to occur to the undesired reflected waves. An alternative light stop is made by depositing an absorptive material on a dielectric mirror for absorbing some of the energy of the undesired modes. Both light stop embodiments are adjustable with respect to a laser wave in a resonant cavity.

BACKGROUND OF THE INVENTION

This invention relates to optical systems and more particularly, theinvention relates to the suppression of higher-order transverse modes ina multi-mode resonant cavity of a ring laser gyroscope.

One of the most significant ring laser gyroscopes yet proposed andconstructed employs four waves of two pairs each propagating in oppositedirections. Such systems are shown and described in U.S. Pat. Nos.3,741,657, 3,854,819 and 4,006,989 to Keimpe Andringa and assigned tothe present assignee. In such laser systems, circular polarization foreach of the four waves is used. The pair of waves, or beams, propagatingin the clockwise direction includes both left-hand circularly polarized(LCP) waves and right-hand circularly polarized (RCP) waves as do thosewaves propagating in the counter-clockwise direction. The four-frequencyor multi-oscillator ring laser gyro provides a means of circumventingthe frequency locking or lock-in problem present in all conventional ortwo-frequency laser gyroscopes. This lock-in phenomenon occurs when twotraveling waves propagating in opposite directions in a resonant cavityat slightly different frequencies are pulled toward each other tocombine in a single frequency standing wave. However, when thefrequencies of the counter-rotating waves are sufficiently separated infrequency, the pulling together does not occur. The four-frequencyapproach may be described as two independent laser gyros operating in asingle stable resonator cavity, sharing a common optical path, butstatically biased in opposite senses by the same passive bias element.In the differential output of these two gyros, the bias then cancels,while any rotation-generated signals add, thereby avoiding the usualproblems due to drifts in the bias and giving a sensitivity twice thatof a single two-frequency gyro. Because the bias need not be dithered,the gyro never passes through lock-in. Hence, there are nodither-induced errors to limit instrument performance. For this reason,the four frequency gyro is intrinsically a low noise instrument, and itis well suited for applications requiring rapid position update or highresolution.

The four different frequencies are normally generated by using twodifferent otical effects. First, a crystal polarization rotator may beused to provide a direction-independent polarization causing theresonant waves to be circularly polarized in two directions. Thepolarization rotation results from the refractive index of the rotationmedium being slightly different for RCP and LCP waves. Alternatively, anon-planar ring path may be used which inherently supports onlycircularly polarized waves without the use of a crystal rotator. Anon-planar electromagnetic wave ring resonator is shown and described inU.S. Pat. No. 4,110,045 to Irl W. Smith, Jr. and Terry A. Dorschner andassigned to the present assignee. Second, a Faraday rotator is used toprovide non-reciprocal polarization rotation, by having a slightlydifferent refractive index for clockwise (cw) traveling waves than forcounter-clockwise (ccw) traveling waves. This causes the cw and ccw RCPwaves to oscillate at slightly different frequencies while the cw andccw LCP waves are similarly but oppositely split. Thus, a laser gyrooperates with right circular polarized waves biased in one direction ofrotation and with left circular polarized waves biased in the oppositedirection, the bias being cancelled by subtracting the two outputs.

In the resonant cavity of a ring laser gyroscope, there are a number ofresonant modes many of which are unwanted and must be suppressed. In theprior art, the suppresson of unwanted modes has been accomplished bymachining a narrow spatial aperture into the gyro block cavity,preferably opposite a spherical mirror in a three mirror cavity. Anotherapproach has been to insert a copper disk with a center hole into aresonant cavity as part of a Faraday rotator assembly. Still anotherapproach, in the prior art has been to rely on the resonant cavity wallimperfections inherently present from the machining process of a lasergyro block. A major drawback of the prior art mode discrimination orsuppression approaches has been that the aperture was not adjustable,thereby preventing fine tuning once the gyro block was machined andassembled; in addition, a scattering of the intercepted light wavesoccurred causing an increase in lock-band occurrences at high annularrotation rates and a variation in the gyro bias. These variationsdegrade the performance of a ring laser gyro.

SUMMARY OF THE INVENTION

This invention discloses an optical system comprising means forproviding a plurality of electromagnetic waves and dielectric materialmeans within an optical path of said optical system for altering thephase and amplitude characteristics of said electromagnetic waves. Thedielectric material means comprises a dielectric material having one ormore areas of said material treated with an electron beam for producingthe phase and amplitude alterations to said waves. The dielectricmaterial comprises a plurality of layers, each of which may have adifferent index of refraction; the layers comprise alternate layers ofsilicon dioxide and titanium dioxide deposited on a fused silicasubstrate. The dielectric material means may also comprise a dielectricmirror with an absorption material deposited on part of said mirror forsuppressing unwanted resonant modes. The thickness of said absorptivematerial increases as a function of the distance away from the center ofsaid mirror to produce an increase in energy loss to said undesiredresonant modes.

This invention further discloses a mode discrimination means forsuppressing undesired resonant modes within a closed path, means forproducing a plurality of counter-traveling electromagnetic waves withinthe closed path, means for producing a direction-dependent phase shiftto said waves resulting in a frequency splitting between saidcounter-traveling and means for adjusting said suppressing means duringthe propagation of said waves.

The invention further discloses a multi-frequency ring laser gyroscopehaving a closed path with a gain medium for the propagation of aplurality of electromagnetic waves in opposite directions, each of saidwaves being of a different frequency, means for producing circularlypolarized counter-traveling waves in said closed path arranged in pairsof first and second polarization sense, means for producing adirection-dependent phase shift to said waves resulting in a frequencysplitting between counter-traveling waves in each of said pairs, andmeans for suppressing undesired resonant modes within said closed path.The circularly polarized counter-traveling wave means comprises anon-planar closed path. The laser gain medium comprises a mixture ofhelium and neon electrically excited by electrodes comprising one ormore anodes and cathodes. A Faraday rotator produces thedirection-dependent phase shifts to the electromagnetic waves and theclosed path comprises means for absorbing electromagnetic wavesreflected from the Faraday rotator.

This invention also discloses a laser gyroscope having a closed pathwith a gain medium for the propagation of a plurality of electromagneticwaves in opposite directions, each of said waves being of a differentfrequency, means for producing circularly polarized counter-travelingwaves in said closed path arranged in pairs of first and secondpolarization sense, means for producing a direction-dependent phaseshift to said waves resulting in a frequency splitting betweencounter-traveling waves in each of said pairs, reflective means fordirecting said electromagnetic waves around said closed path, and atleast one of said reflective means comprising adjustable, modediscriminating means for suppressing undesired resonant modes withinsaid closed path.

In one embodiment of mode discrimination in a ring laser gyroscope, themode suppressing means comprises a phase sensitive light stop. Thislight stop further comprises a dielectric mirror partially treated withan electron beam for producing a phase and amplitude change to a part ofthe electromagnetic waves reflected from said treated part of saiddielectric mirror. The dielectric mirror comprises a plurality ofalternate layers of a high and low index of refraction dielectricmaterial including silicon dioxide and titanium dioxide deposited on afused silica substrate.

In another embodiment of mode discrimination in a ring laser gyroscope,the mode suppressing means comprises an absorptive light stop. Thislight stop comprises a dielectric mirror with an absorptive materialdeposited on part of the mirror. The mirror is made from alternatelayers of silicon dioxide and titanium dioxide deposited on a fusedsilica substrate. The thickness of the absorptive material deposited onthe surface of the mirror increases as a function of distance away fromthe center of the mirror producing a differential loss to the undesiredresonant modes. Light absorbing glass is one type of absorptive materialsuitable for a light stop.

The invention further discloses the method of suppressing undesiredresonant modes within a laser gyroscope system comprising the steps ofpropagating a plurality of counter-traveling electromagnetic waveswithin a closed path each of said waves being of a different frequency,amplifying said waves in a gain medium which is common to at least aportion of said path of each of said waves, providing reciprocalpolarization dispersive means and non-reciprocal polarization dispersivemeans for said waves in said closed path, and providing discriminatingmeans on at least one of a plurality of reflectors within said closedpath for suppressing undesired resonant modes. The step of providingmode discriminating means for suppressing the undesired resonant modescomprises either a phase sensitive light stop or an absorptive lightstop.

BRIEF DESCRIPTION OF THE DRAWINGS

Other and further features and advantages of the invention will becomeapparent in connection with the accompanying drawings wherein:

FIG. 1 is a block diagram of a ring laser gyroscope system incorporatinga perspective view of the gyro block embodying the invention;

FIG. 2 is a graph showing the gain versus frequency characteristics ofthe ring laser gyroscope system of FIG. 1 indicating the relativepositions of the frequencies of the four waves within the system;

FIG. 3A is a front surface view of a reflector embodying the inventionof a phase sensitive light stop as viewed from inside the closed path asshown in FIG. 1;

FIG. 3B is an exaggerated side elevation cross-sectional view of areflector embodying the invention of a phase sensitive light stop;

FIG. 4A is a front surface view of a reflector embodying the inventionof an absorptive light stop as viewed from inside the closed path asshown in FIG. 1;

FIG. 4B is an exaggerated side elevation cross-sectional view of areflector embodying the invention of an absorptive light stop;

FIG. 5A is a graph showing the fundamental mode of the Hermite GaussianFunction, U₀ (ξ), representing a one dimensional intensity distributionof the fundamental mode;

FIG. 5B is a graph of the Hermite Gaussian Function, U₁ (ξ),representing a one dimensional intensity distribution of the firstoff-axis mode;

FIG. 5C is a graph of the Hermite Gaussian Function, U₂ (ξ),representing a one dimensional intensity distribution of the secondoff-axis mode; and

FIG. 5D is a graph of the Hermite Gaussian Function; U₃ (ξ),representing a one dimensional intensity distribution of the thirdoff-axis mode.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a ring laser gyro block 10 is shown. Containedwithin a gyro block 10 are a non-planar resonator cavity 16 whichprovides a closed path for the propagation of electromagnetic waves,four dielectric mirrors or reflectors 13, 30, 32 and 38 for directingthe waves around the closed path, a Faraday rotator assembly 28 whichprovides for non-reciprocal polarization rotation of the propagatingwaves, anodes 14 and 36, cathode 34 and a laser gain medium 39 withinoptical cavity 16 having a helium-neon gas mixture where the two activeisotopes are neon 20 and neon 22. The gaseous gain medium 39 iselectrically excited by discharge currents generated between the anodes14 and 36 and cathode 34, and it becomes a light emitting laser gainmedium or plasma sustaining the resonant laser waves in the opticalcavity 16. The gyro block 10 is preferably constructed with a materialhaving a low thermal coefficient of expansion such as a glass-ceramicmaterial to minimize the effects of temperature change upon a lasergyroscope system. A preferred commercially available material is soldunder the name of Cer-Vit™ material C-101 by Owens-Illinois Company;alternatively, Zerodur™ by Schott Optical Company may be used.

The non-planar ring inherently supports only circularly polarized waveswithout the use of crystal rotator. The placement of reflectors 13, 30,32 and 38 in the ring path 16 produces a phase alteration which altersthe resonant frequencies of the waves. The result, as shown in FIG. 2,is that the waves of left-hand circular polarization (f₁ and f₂) willhave a resonant frequency different from the resonant frequency of theright-hand circular polarized waves (f₃ and f₄). This non-planarelectromagnetic wave ring resonator is shown and described in U.S. Pat.No. 4,110,045 to Irl W. Smith, Jr. and Terry A. Dorschner and assignedto the present assignee and the specification is incorporated herein byreference.

Reflector 13 is attached to a piezoelectric element 12 which moves thereflector in and out as part of a cavity path length control system.Reflector 30 is used solely for reflecting the electromagnetic waves inthe closed laser path. Reflector 32 in addition to reflecting thedesired electromagnetic waves within the closed laser path 16 comprisesthe invention described herein of a light stop apparatus 33 forsuppessing undesired resonant modes within said closed path. Reflector38 is only partially reflective thereby allowing a small portion of thewaves incident on its surface to pass through the reflector and becombined and processed to provide rotational information.

The Faraday rotator assembly 28 is shown in one of the segments of thenon-planar ring laser path 16 between reflectors 30 and 32. Thisnon-reciprocal magneto-optical device produces a phase delay bias forwaves of either circular polarization sense traveling clockwise which isdifferent from that for waves of similar polarization travelingcounterclockwise. The combination of reflectors 13, 30, 32 and 38 andthe Faraday rotator 28 is such that the ring resonator supports waveshaving frequencies of oscillation as shown in FIG. 2. However, there areother alternative means for accomplishing the same results as theFaraday rotator. One such means utilizing the Zeeman effect is describedin U.S. Pat. No. 4,229,106 to Terry Dorschner et al and assigned to thepresent assignee.

Photon absorbers 24 and 26 are positioned for absorbing reflectedelectromagnetic waves from the Faraday rotator assembly 28. They arefully described in U.S. patent application Ser. No. 235,320 (RaytheonDisclosure No. 33026) filed in the U.S. Patent Office on Feb. 17, 1981by Michael Perlmutter and Lawrence L. Clampitt and assigned to thepresent assignee.

In addition to the laser gyroscope block 10, FIG. 1 shows connections toassociated laser gyroscope electronics and optics. The high voltagepower supply 50 provides a high negative voltage to the cathode 34 and ahigh positive voltage to the piezoelectric driver 52. Discharge controlelectronics 54 provides regulation of the current flowing from theanodes to the cathodes; different gyro blocks may require differentvalues of cathode current depending on the optical losses within theparticular gyro block.

The path length control system is a feedback network which maintains aconsistent and optimum optical path length within the gyro cavity 16. Itcomprises the detector preamplifier 56 path length control 58 and highvoltage piezoelectric driver 52 electronics. The optical path length iscontrolled by means of a reflector 13 which is mounted on apiezoelectric transducer (PZT) 12. The high voltage driver 52 operatesthe PZT 12 with an applied voltage ranging from 0 volts to 400 volts.Since stable operating points or modes occur at path length intervalsequal to one-half the laser wavelength, the mode which is closest to thecenter of the transducer's dynamic range will normally be chosen as apermanent operating point. The detector preamplifier 56 separates the ACsignals and DC signals received from the output optics 35. The DCsignals are used for the path length control. The AC signals are sinewaves representing the gyro output, and they are sent to the signalprocessor 60 where they are converted into two digital pulse streams (f₁-f₂ and f₃ -f₄) with one pulse produced for each cycle within theincoming voltage waveforms. The pathlength control is fully described inU.S. Pat. No. 4,108,553 to Albert N. Zampiello and Bradley J. Patch, Jr.and assigned to the present assignee. The specification of this patentbeing incorporated herein by reference.

The output optics 35 extracts a portion of each beam circulating withinthe laser cavity to produce the two output signals, f₁ -f₂ and f₃ -f₄,each one of which represents the difference in frequency between wavepairs having the same sense of circular polarization within the cavityas shown in FIG. 2. The output reflector 38 has a transmission coatingon one side and a beamsplitter coating on the other side. Both coatingsare a standard type using alternate layers of TiO₂ and SiO₂. Thebeamsplitter coating transmits half the incident intensity and reflectsthe other half. A retro-reflecting prism 37 is used to heterodyne thetwo beams. This right angle prism is made of fused quartz and hassilvered reflective faces. A dielectric coating is used between thesilver and fused quartz to obtain minimal phase error upon reflection. Aquarterwave plate (not shown) followed by sheet polarizers are used toseparate the four frequencies present in each beam. A wedge (not shown)is used between the retro-reflecting prism and the quarterwave plate toprevent reflections from the interfaces from propagating back into thegyro cavity and mixing with the counter-rotating beams. A photo-diodecover glass (anti-reflection coated on one side) and a photo-diodepackage (not shown) complete the output optics 35. An optical cement isused between the various interfaces to provide adhesion and to minimizereflections. The output optics is fully described in U.S. Pat. No.4,141,651 to Irl W. Smith and Terry A. Dorschner and assigned to thepresent assignee, the specification of this patent being hereinincorporated by reference.

Referring now to FIG. 3A, reflector 32 is shown which functions as aphase sensitive light stop apparatus comprising a dielectric mirror 31with a specific area of the mirror 31 irradiated by an electron beamfrom a scanning electron microscope or other similar instrument. Thiselectron beam treated area 33 creates a phase shift and a smallamplitude reduction to some fraction of a traveling electromagnetic waveas a result of a change in the index of refraction in the electron beamtreated area 33. The dielectric mirror 31 as shown in FIG. 3B is madefrom alternate layers of silicon dioxide (SiO₂) 62 and titanium dioxide(TiO₂) 64 on a fused silica SiO₂ substrate 66. The treated area 33extends through most of the alternate layers of SiO₂ and TiO₂. Thisphase sensitive light stop provides resonant mode discrimination withoutadding any measurable wave scattering to the counter-rotatingelectromagnetic waves. It provides a small amount of electromagneticwave amplitude reduction but the effect is not sufficient by itself tosuppress unwanted modes. However, because it also changes the phase of asmall fraction of the unwanted counter-rotating waves (high order modes)within the closed path 16, they acquire sufficient loss to prevent themfrom lasing.

The irradiation of a reflector or dielectric mirror 31 in order toproduce a treated area of approximately 4 millimeters long and 0.5millimeters wide that will cause a phase shift of an electromagneticwave is accomplished by using an electron beam instrument such as ascanning electron microscope (SEM) manufactured by Cambridge ScientificInstrument Ltd. of Cambridge, England, Model S-4 Stereoscan with VideoPresentation Unit (VPU). The SEM controls are adjusted as follows duringthe electron beam treating procedure:

Accelerating voltage (E_(B))=30K Volts

Specimen Current (I_(S))=2×10⁻⁸ Amperes

Final Apperature diameter=700 micrometers

Magnification=20×

Mode=Single Line Repetitive Scan

Tilt=Zero

The procedure for treating a dielectric mirror with the SEM is asfollows:

1. Coat a dielectric mirror with 500 angstroms of copper for connectionto ground.

2. Adjust the SEM for normal specimen viewing at a zero tilt angle andrecord the working distance reading.

3. Set the video presentation unit (VPU) of the SEM for a rotation angleof zero at the working distance determined during Step 2.

4. Position the desired region of the dielectric mirror to be treatdunder the electron beam.

5. Defocus the electron beam to a working distance of 40 millimeters.

6. Set the VPU for a rotation angle of zero at a working distance of 40millimeters.

7. Set Mode to Line Scan.

8. Expose the dielectric mirror to the electron beam for four (4) hours.

9. Strip the copper from the dielectric mirror in a suitable etchantsuch as ammonium persulphate.

Referring now to FIGS. 5A-5D, Hermite-Gaussian Functions are shownrepresenting the one dimensional intensity distribution of thefundamental and higher order off-axis resonant modes. The existence ofthese modes in a resonator type structure such as a confocalFabrey-Perot type resonator or a ring laser resonator with spherical andflat reflectors of equal size and reflectivity has been demonstrated anddescribed in detail in the articles "Resonant Modes in a MaserInteferometer" by A. G. Fox and Tingye Li and "Confocal MultimodeResonator for Millimeter Through Optical Wavelength Masers" by G. D.Boyd and J. P. Gordon in the Bell System Technical Journal, March 1961,Volume 40, pp. 453-488 and pp. 489-508, respectively. A mode may bedefined as a field distribution that reproduces itself in spatialdistribution and phase, though not in amplitude, as the wave bouncesback and forth between the two reflectors. Because of losses due todiffraction and reflection, the reproduced pattern is reduced inintensity on each succeeding traversal of the resonator if no gainmedium is present. In the aforementioned articles, the authors haveshown that there is a set of modes which will reproduce themselves overthe equal size mirrors of the resonator. When the effect of diffractionlosses due to finite apertures is included, the modes become unique andeach mode has its own characteristic rate of decay or Q. When gainsupplied by a helium-neon discharge is included, a steady statecondition is reached where all the modes that have more gain than losswill oscillate or lase. Said losses of a mode comprises diffraction lossand losses due to imperfect mirrors.

For the case of low diffraction losses, the eigenfunctions of the modesare still given with good approximation by the following Hermit-Gaussianfunctions, as shown in FIGS. 5A-5D, which are exact only for theloss-less case of infinite apertures:

    U.sub.L =(π.sup.1/2 L!2.sup.L).sup.-1/2 H.sub.L (ξ)e.sup.-ξ.spsp.2.sup./2

where, U_(L) =Hermite-Gaussian Polynomial of Order L

The curves in FIGS. 5A-5D show the intensity distribution of the loworder transverse electric modes which are normalized to present a fixedamount of total beam power in all the modes ##EQU1## It is important tonote that the higher order modes contain more energy in the "tails"(greater distance from the beam center) than do lower order modes. Thelight stop mode discrimination invention adds sufficient energy loss tothe higher order modes to prevent them from lasing, but does not addenough loss to the fundamental to prevent it from lasing.

Referring now to FIGS. 4A and 4B, an alternate absorptive light stop 70is shown. It is made by depositing an absorptive material 74 on adielectric mirror 72. The absorptive material, such as sputtered lightabsorbing glass, is deposited on top of approximately 20 alternatelayers of SiO₂ and TiO₂ which have been deposited on a fused silica SiO₂substrate 76. The absorptive material 74 thickness varies linearly orquadratically as a function of distance from the center of the mirror 72in order to minimize scattering effects and achieve higher order modesuppression by energy absorption. However, with this type of light stop,there will always be some type of discrete interface 75 present betweenthe start of the absorptive material (closest to the center of themirror 72) and the surface of the mirror 72, which produces theundesirable scattering of some amount of incident light energy.

The light stop invention provides a significant improvement over theprior art methods of resonant mode suppression by the ability to adjustthe light stop after fabrication and assembly of a gyro block 10 andduring the propagation of electromagnetic waves within the closed path16 as shown in FIG. 1. This adjustment is accomplished by changing theposition of a reflector 32 with respect to the traveling waves. If thesurface of a reflector is flat, the adjustment procedure is simply oneof sliding the reflector on the mounting surface while monitoring thelosses of the fundamental and higher-order modes at the output of thesignal processor 60. If a reflector is spherical (not shown) andcomprises an absorptive light stop 70, then the absorptive material mustbe deposited in a radial direction and mode adjustment would beaccomplished by rotating the reflector along its spherical axis. Inaddition to the adjustability improvement, this light stop inventioneliminates the need for an intracavity element to perform undesirablemode suppression.

This concludes the description of the embodiments of the inventiondescribed herein. However, many modifications and alterations will beobvious to one skilled in the art without departing from the spirit andscope of the inventive concept. Therefore, it is intended that the scopeof this invention be limited only by the appended claims.

What is claimed is:
 1. In combination:an electromagnetic wave ringresonator for supporting an electromagnetic wave having a fundamentalresonant mode disposed along an axis of said resonator and higher orderresonant modes displaced from said axis; means disposed in saidresonator having absorptive material displaced from said axis forabsorbing energy of said higher order resonant modes; and saidabsorptive material means comprising a dielectric mirror with anabsorptive material deposited on at least a portion of said mirror forsuppressing said unwanted resonant modes.
 2. A ring laser gyroscopecomprising:means for producing a plurality of counter-travelingelectromagnetic waves within a closed path with a gain medium;absorptive material means within said closed path for suppressingunwanted resonant modes of said electromagnetic waves; and saidabsorptive material means comprising a dielectric mirror with anabsorptive material deposited on at least a portion of said mirror forsuppressing said unwanted resonant modes.
 3. The combination as recitedin claim 2 wherein:the thickness of said absorptive material increasesas a function of the distance away from the center of said mirror toproduce an increase in energy loss to said undesired resonant modes. 4.The combination as recited in claim 2 wherein:said absorptive materialdeposited on part of said mirror comprises a light absorbing glass. 5.The combination as recited in claim 2 wherein:said dielectric mirrorfurther comprises a plurality of alternate layers of silicon dioxide andtitanium dioxide deposited on a fused silica substrate.
 6. Incombination:means for producing a plurality of counter-travelingelectromagnetic waves within a closed path with a gain medium; means forproducing a direction-dependent phase shift to said waves resulting in afrequency splitting between said counter-traveling waves; and means forsuppressing undesired resonant modes within said closed path, saidsuppressing means comprising an absorptive light stop having adielectric mirror with an absorptive material deposited on at least aportion of said mirror.
 7. The combination as recited in claim 6wherein:said gain medium comprises a mixture of helium and neonelectrically excited by electrodes; and said electrodes further compriseone or more anodes and cathodes for producing an excitation current. 8.The combination as recited in claim 6 wherein:said closed path furthercomprises a plurality of reflectors for directing said electromagneticwaves around said path.
 9. The combination as recited in claim 6wherein:said direction-dependent frequency splitting means comprisesmagneto-optical means.
 10. The combination as recited in claim 6wherein:said suppressing means further comprises means for adjustingsaid suppressing means during the propagation of said waves.
 11. Thecombination as recited in claim 6 wherein:said undesired resonant modeswithin said closed path comprises off-axis higher order resonant modes.12. In combination:a laser gyroscope having a closed path with a gainmedium for the propagation of a plurality of electromagnetic waves inopposite directions, each of said waves being of a different frequency;means for producing circularly polarized counter-traveling waves in saidclosed path arranged in pairs of first and second polarization sense,said circularly polarized counter-traveling waves means comprising anon-planar closed path; mens for producing a direction-dependent phaseshift to said waves resulting in a frequency splitting betweencounter-traveling waves in each of said pairs; and mens for suppressingundesired resonant modes within said closed path, said suppressing meanscomprising an absorptive light stop having a dielectric mirror with anabsorptive material deposited on at least a portion of said mirror, thethickness of said absorptive material increasing as a function of thedistance away from the center of said mirror to produce an increase inenergy loss to said undesired resonant modes; and said dielectric mirrorfurther comprising a plurality of alternate layers of silicon dioxideand titanium dioxide deposited on a fused silica substrate.
 13. Thecombination as recited in claim 12 wherein:said laser gain mediumcomprises a mixture of helium and neon electrically excited byelectrodes; and said electrodes further comprise one or more anodes andcathodes for producing an excitation current.
 14. The combination asrecited in claim 12 wherein:said closed path further comprises aplurality of reflectors for directing said electromagnetic waves aroundsaid path.
 15. The combination as recited in claim 12 wherein:saidundesired resonant modes within said closed path comprises off-axishigher order resonant modes.
 16. In combination:a laser gyroscope havinga closed path with a gain medium for the propagation of a plurality ofelectromagnetic waves in opposite directions, each of said waves beingof a different frequency; means for producing circularly polarizedcounter-traveling waves in said closed path arranged in pairs of firstand second polarization sense; means for producing a direction-dependentphase shift to said waves resulting in a frequency splitting betweencounter-traveling waves in each of said pairs; reflective means fordirecting said electromagnetic waves around said closed path; and atleast one of said reflective means comprising adjustable, modediscriminating means for suppressing undesired resonant modes withinsaid closed path, said adjustable mode discriminating means having anabsorptive material deposited on at least a portion of said one of saidreflective means.
 17. The combination as recited in claim 16wherein:said laser gain medium comprises a mixture of helium and neonelectrically excited by electrodes; and said electrodes further compriseone or more anodes and cathodes for producing an excitation current. 18.The combination as recited in claim 16 wherein:said reflective meanscomprises a plurality of dielectric mirrors.
 19. The combination asrecited in claim 16 wherein:said circularly polarized counter-travelingwave means comprises a non-planar closed path with right circularlypolarized waves and left circularly polarized waves propagating in saidpath.
 20. The combination as recited in claim 16 wherein:saiddirection-dependent frequency splitting means comprises a Faradayrotator.
 21. The combination as recited in claim 16 wherein:said modediscriminating means for suppressing said undesired resonant modescomprises an absorptive light stop.
 22. The combination as recited inclaim 16 wherein:said mode discriminating means comprises a dielectricmirror with said absorptive material deposited on a portion of saidmirror.
 23. The combination as recited in claim 22 wherein:the thicknessof said absorptive material increases as a function of the distance awayfrom the center of said mirror to produce a differential loss to saidundesired resonant modes.
 24. The combination as recited in claim 22wherein:said absorptive material deposited on part of said mirrorcomprises a light absorbing glass.
 25. The combination as recited inclaim 22 wherein:said dielectric mirror further comprises a plurality ofalternate layers of silicon dioxide and titanium dioxide deposited on afused silica substrate.
 26. The method of suppressing undesired resonantmodes within a laser gyroscope system comprising the stepsof:propagating a plurality of counter-traveling electromagnetic waveswithin a closed path each of said wves being of a different frequency;amplifying said waves in a gain medium which is common to at least aportion of said path of each of said waves; and depositing absorptivematerial means on at least a portion of one of a plurality of reflectorswithin said closed path for suppressing said undesired resonant modes.27. The method as recited in claim 26 wherein:said step of depositing anabsorptive material means comprises making the thickness of saidabsorptive material increase as a function of distance away from thecenter of said one of a plurality of reflectors to produce an increasein energy loss to said undesired resonant modes.
 28. The method asrecited in claim 27 wherein:said one of a plurality of reflectorscomprises a plurality of alternate layers of silicon dioxide andtitanium dioxide deposited on a fused silica substrate.